It is known that the polyimide film has excellent characteristics in heat resistance, cold resistance, chemical resistance, electric insulation, mechanical strength, etc., and the polyimide film is broadly utilized in electric insulating material of electric wires, adiabatic material, base film of flexible printed-circuit boards (FPC), carrier tape film for tape automated bonding (TAB) of IC, tape for fixing lead frames of IC, etc. Among them, in the usages such as FPC, carrier tape for TAB, and tape for fixing leads, the polyimide film and a copper foil are usually adhered via various adhesives.
An important practical characteristic of the polyimide film, which is required for these usages, is a slip characteristic (easy slip characteristic) of said film. However, in reality, a sufficiently satisfactory slip characteristic in the automatic optical inspection system (AOI) cannot be provided.
In other words, in various film working processes, it has been known that the operability and the handling characteristic are improved by securing the easy slip characteristic of a film support (for example, roll) and a film or the easy slip characteristic of films, so that the generation of inferior portions such as folds on the film can be avoided. On the other hand, in a flexible printed-circuit board as the main usage of the polyimide, the polyimide is usually adhered to a copper foil via various kinds of adhesives, and the adhesion of the polyimide with the copper foil is insufficient in terms of its chemical structure and chemical resistance (solvent stability) in many cases. It is known that the polyimide is adhered after a surface treatment (alkali treatment, corona treatment, plasma treatment, sandblast treatment, etc.).
However, in the fine pitch formation of recent electronic components, the inspection of line width, foreign matters, etc., with the naked eyes has been mainly carried out especially in the inspection of FPC. However, after the introduction of the automatic optical inspection system (AOI), even if a sufficiently satisfactory running characteristic is obtained in heat-resistant films of this conventional technical method, inorganic powders are too large in the AOI, so that said inorganic powders are considered as foreign matters in comparison with the recent narrow pitch formation of FPC, etc., causing a large obstacle to the automatic optical inspection system.
Known specifically as conventional easy slipping techniques in the polyimide are, for example, a method that adds an inactive inorganic compound (for example, orthophosphate of alkaline-earth metals, calcium secondary phosphate anhydride, calcium pyrophosphate, silica, and talc) to polyamic acid (see patent reference 1), and a method that makes many projections consisting of inorganic particles, in which part of inorganic particles with an average particle diameter of 0.01-100 μm are embedded and held in a polyimide surface layer and partially exposed, exist at 1×10-5×108 pieces/mm2 on the surface layer of said film (see patent reference 2). In this method, the friction coefficient of the film surface is lowered by aggressively exposing the projections to the surface, so that an easy slip effect is effectively obtained. However, in this method, since the inorganic particles are partially exposed, scratches are generated on the film surface, causing appearance inferiority, and in the automatic optical inspection system, the particle diameter is too large, so that the particles are considered as foreign matters. In this technique using inorganic particles, fillers are lost when holes are opened by laser, etc., leading to a lowered working precision.
In addition, also known as another technique is a polyimide film (patent reference 3) in which total aromatic polyimide particles with a median diameter of 0.3-0.8 μm and a maximum diameter of 2 μm or smaller composed of polyimide containing 80% or more pyromellitic acid and p-phenylenediamine components are dispersed at a ratio of about 0.5-10 mass % to the polyimide of a polyimide surface layer into said polyimide surface layer of 1 μm. However, in this technique, the particle diameter is too large in both the average particle diameter and the median diameter, so that the polyimide particles are considered as foreign matters in a test of the automatic optical inspection system (AOI), and if the polyimide particles are dispersed into only the surface layer of 1 μm, the cohesion of the particles is likely to be caused, which is not preferable.